1. Field of the Invention
The invention generally relates to projection image display apparatuses and polarization converters. More particularly, the invention relates to a projection image display apparatus including an illumination optics incorporating a polarization converter and wave difference plates.
2. Background Art
Projection image display apparatuses (projectors) have been used for purposes of conducting enlarged projections of presentation documents in the course of meeting and lecture. Since the projection image display apparatuses of such use are often subjected to bright indoor environments unlike for home use, efforts have been in progress for increasing luminance.
In addition, miniaturization of the apparatuses is a general trend for achieving low costs, and the luminous density in optical system has been ever increasing, as a result. While the amount of air flow for cooling may need to increase with such increase in luminous density, it is desirable to obtain quieter performance even for the apparatuses of high luminance. In order to align the state of light polarization efficiently in such projection image display apparatuses, a PS converter (polarization conversion element) has been used.
Such a PS converter is configured to separate P-polarized light and S-polarized light with a PBS (polarization beam splitter) film, and allow one of the P-polarized light and the S-polarized light to pass through a half-wave plate while rotated by 90 degrees, whereas the direction of light polarization is aligned in a single direction. Although a polycarbonate film is generally used for forming the half-wave plate, degradation of its optical characteristic caused by yellowing or burnt may become a problem with the increase in the light density.
In order to improve durability and heat-resistance, a PS converter using a wavelength plate of quartz in place of the polycarbonate film is disclosed in Japanese Unexamined Patent Application Publication No. 2003-302523. Since such a wavelength plate is designed to yield a phase difference of λ/2 at a certain wavelength, its effective range of wavelength is relatively narrow in general.
In order to increase the effective wavelength range, a technology of laminating a plurality of wavelength plates after suitably aligning their directions is disclosed to be known previously (described in Crystal Optics, the first edition in 1975, which is edited by Optics Forum of the Japan Society of Applied Physics, and published by Morikita Shuppan Co., Ltd., Japan). A quartz wavelength plate using such technologies is also disclosed in Japanese Unexamined Patent Application Publication No. 2004-170853.